This invention relates generally to semiconductor processing and more particularly to industry standard wafer cassettes that are employed to hold twenty-five standard silicon or gallium arsenide wafers of a certain diameter. The semiconductor industry uses these thin, very fragile silicon or gallium arsenide wafers that are typically 4, 5, 6 or 8 inches in diameter on which to fabricate integrated circuits. More than $100,000 worth of packaged integrated circuits may be fabricated on a single one of these wafers. It is therefore extremely important to prevent damage to the fragile wafers on which as many as 300 separate manufacturing processes may be performed before the wafer is cut into separate dies for final packaging. Several types of wafer cassettes are used to hold these wafers, depending on the particular process being performed. For example, teflon cassettes are typically employed to hold the wafers while they are in chemical prosesses, ABS plastic traveller cassettes are employed to transport wafers from one processes to the next, and quartz cassettes are employed to retain wafers during high temperature processes. The processing equipment required to manufacture integrated circuits is very expensive; so any increase in manufacturing speed and yield is highly desirable.
In order to increase manufacturing speed and reliability, loading and unloading robots are used to transfer the wafers from one cassette to another. However, wafers are sometimes loaded into incoming cassettes by hand, typically from manual inspection stages. Manual loading of cassettes presents the opportunity for creating a disastrous condition known as cross slotting. This condition occurs when a wafer is not loaded into corresponding slots on each side of the cassette, resulting in one side of a loaded wafer being higher than the other side, rather than the wafer being in a perfectly horizontal position. When a cross slotted wafer is presented to the pickup arm of a robot, the arm jams one side of the fragile wafer, thereby not only destroying that wafer but also damaging other wafers in the cassette as the result of contamination of those otherwise good wafers by the pieces of the shattered wafer.
An additional problem is presented in prior art cassette handling systems when a cassette coming into a robot stage is only partially filled with wafers. The robot assumes that every slot of a cassette contains a wafer and is therefore designed to traverse each of the twenty-five slot positions of the cassette and to attempt to pick a wafer from each slot position. In the case of wafer cassettes that are not completely filled, valuable time is wasted as the robot searches unfilled wafer positions.
Several attempt have been made in the prior art to provide wafer detection systems. One such prior art system is based on non-inductive proximity detection, since wafers are not ferromagnetic. In this type of system, a capacitive field effect sensor is mounted on the robot arm to detect the presence or absence of a wafer in each slot position of a cassette. These wafer detection systems suffer from long traverse times and, more importantly, make no provision for cross slotted wafers.
Another known wafer detection system employs a video camera in which a small video head is remotely mounted on the robot arm. A video framegrabber and analysis software are integrated with the robot controller. The images obtained by the remote head video camera ar used to detect the presence or absence of a wafer in each slot position of a cassette. These video camera systems are expensive, and, like the proximity detectors described above, have no capability for detecting cross slotted wafers.
It is therefore a principal object of the present invention to provide a semiconductor wafer cassette mapper that is capable of rapidly and accurately providing information representative of the presence or absence of a wafer in each slot of a wafer cassette, thereby eliminating the need for a robot wafer handler to individually traverse each of the slots in order to make that determination.
It is a further object of the present invention to provide a semiconductor wafer cassette mapper that detects cross slotted wafers, thereby eliminating the risk of damage to a a cross slotted wafer, other wafers in the cassette, the cassette itself, and the clean room environment generally, that would otherwise result from attempted retrieval of the cross slotted wafer by the arm of a robot wafer handler.
These and other objects are accomplished in accordance with the illustrated preferred embodiment of the present invention by providing a base member for receiving a standard slotted wafer cassette, the base member including transmitter and receiver modules on opposite sides thereof. The transmitter and receiver modules include like pluralities of inwardly facing, aligned light apertures. Each of the light apertures in the transmitter module contains a light emitting infrared transmitter, and each of the light apertures in the receiver module contains an infrared receiver. Control circuitry selectively activates the light emitting transmitters for a predetermined period of time and selectively interrogates the infrared receivers during that period of time to determine if a particular receiver has received light transmitted by a selected transmitter. This information is then processed by logic circuitry to determine the presence or absence of a semiconductor wafer in a particular corresponding pair of slots of the wafer cassette and to detect the existence of cross slotted condition in which a particular semiconductor wafer is not aligned in a corresponding pair of slots of the wafer cassette. Several wafer cassette mappers may be connected in a system configuration to serve a semiconductor fabrication process. In such a system configuration, an external host computer is employed to control the various wafer cassette mappers and to enable the user to request cassette map information from selected ones of the wafer cassette mappers and to generate a visual display or hard copy of the requested cassette map information.